Add additional null check in _cat/shards

The target of the func lambda may be null (e.g. in a mixed cluster
where older nodes lack some of the values)

Relates: #33858 / 331caba
Closes #33877

Author: tvernum

server/src/main/java/org/elasticsearch/rest/action/cat/RestShardsAction.java

@@ -194,7 +194,9 @@ protected Table getTableWithHeader(final RestRequest request) {
     private static <S, T> Object getOrNull(S stats, Function<S, T> accessor, Function<T, Object> func) {
         if(stats != null) {
             T t = accessor.apply(stats);
-            return func.apply(t);
+            if (t != null) {
+                return func.apply(t);
+            }
         }
         return null;
     }